1. Field of the Invention
The present invention relates to hard disk drives. More particularly, the present invention relates to a disk drive employing a method of maintaining a two-tiered defect list comprising a first tier of marginal data sites and a second tier of reserved data sites.
2. Description of the Prior Art
Hard disk drives store large volumes of data on one or more disks mounted on a spindle assembly. Disk drives employ a disk control system for interfacing with a host computer to control the reading and writing of data on a disk. Each disk includes up to two disk surfaces which are capable of storing data. On each disk surface, user data is stored in concentric circular tracks between an outside diameter and an inside diameter of the disk. Servo systems are employed to maintain alignment of a transducer head with a desired target data track (termed "track following") for reading and writing user data on the disk surface within desired control parameters.
Embedded servo systems store servo data on the same disk surface as user data to provide control signals and information employed in the operation of the servo system. User data on the disk surface is divided into groups of data sectors. Embedded servo information is recorded in servo sectors placed in radially continuous narrow wedges between the groups of data sectors. In this regard, servo sectors are commonly referred to as "servo wedges". For example, a concentric data track may include 60 equally spaced servo sectors with data regions (i.e., a region containing data sectors, groups of data sectors or partial data sectors) located between adjacent servo sectors.
As a result of the manufacturing process, defective data sites may exist on the disk surfaces of the disk drive. These defective data sites are termed "prior defects". A defect discovery procedure is performed to locate these defects and mark them out as defective locations on the disk surface which are not available for use. Defect discovery for locating defective data sites is performed after the servo sectors have been written on the disk surface. A typical defect discovery procedure includes writing a known data pattern to the disk surface and subsequently reading the data pattern from the disk surface. Defective data sites are identified by comparing the data pattern read from the disk surface with the known data pattern written to the disk surface.
In order to statistically reduce the chances of encountering a defective data site after the manufacturing process (and thereby reducing hard errors reported by the disk drive), disk drive manufacturers commonly pad the defective data sites. The phrase "pad the defective data site" refers to the procedure of marking out data areas which surround or bound the defective data site as also unusable for storing data. Depending on the size of the defective data site, the defective data site may be "padded" both circumferentially and/or radially. For example, a relatively large defective data site may be due to a scratch on the disk surface. The scratch can be circumferentially padded by adding pads to each end of the scratch. The scratch is radially padded by marking off data sites located on tracks in close proximity to the scratch as unusable. If several defective data sites are detected in close linear proximity to each other, a fill procedure is used to pad or "fill in" the areas between the defective data sites. Similarly, if a cluster of defective data sites are found on the disk surface, the cluster of defective data sites are bounded with pads both radially and circumferentially. Alternatively, a very small defective data site may not be padded.
Defective data sites are padded to improve the quality of a disk drive by reducing the number of hard errors reported due to the defective data sites. For example, it is desirable to bound defective data sites with pads because a defect (such as a scratch) might cause marginal error performance in data sites close to the defective data sites even though they were not detected during the defect discovery procedure.
Defective data sites encountered after formatting the disk surface may be termed "grown" defects. Grown defects are also listed in a table. Most grown defects occur in locations adjacent to defective data sites found during defect discovery. Therefore, padding of the defective data sites reduces the number of grown defects encountered after formatting the disk surface, improving the quality of the disk drive and reducing the chances of reporting a "hard error" as a result of the grown defect.
Following the defect discovery procedure, defective data sites and pad locations are put in a prior defect list which is stored in a table. The prior defect list is used during formatting of the disk surface to generate a defect management table. Within the defect management table, the defective data sites and padded locations may be mapped to data sector locations (cylinder number, head number, and data sector number). Once identified in the defect management table, the defective and padded data sectors may not be used for storing data.
The number of sites marked out on a disk drive as "defective data sites" is used as a measure of quality of the disk drive. Upon interrogation by a host, the disk drive will report the defect list generated in the defect management table, the reported defect list including both defective and padded data sectors. However, due to aggressive padding around defective data sites, the defect list can mischaracterize the quality of the disk surface in the disk drive.
U.S. Pat. No. 5,646,923 to Shea (the '923 Patent) discloses selectively "hiding"--i.e. not disclosing to a host computer--the number and locations of marginally defective areas of a disk identified during formatting of the disk surface. According to the '923 patent, a surface analysis test is performed to generate a list of surface defects (prior defect list) to identify locations and size of defective areas on the disk surface. The list of surface defects is used while formatting the disk surface to generate a defect list identifying defective data sectors which are not usable for storing data. The '923 patent relies on using ECC to detect and subsequently correct errors in order to characterize the marginality of a data sector while formatting the disk surface. The ECC correction test can only find data sites which are presenting errors at the time of formatting and cannot completely forecast all sites which may be prone to grown defects later in the life cycle of the disk drive.
It is desirable to aggressively pad (i.e. add more pads around) a defective data site identified in manufacturing during a defect discovery procedure to reduce the chances of a hard error being reported upon later discovery of a grown defect. There is therefore a need to provide a method to conservatively and aggressively pad defective areas in a disk drive while preserving an accurate representation of the disk drive's surface map defect.